Semiconductor wafer processes commonly involve the exposure of surfaces of the wafers to processing gas in a processing chamber. Many of these processes treat the wafers in batch processes that simultaneously expose a plurality of wafers to the processing gas. In many typical batch treatment processes, the wafers are carried in boats that support the wafers in an array in which they are arranged in parallel rack-like structure. These boats include those in which horizontally oriented wafers are spaced in a vertical stack. A common boat may be formed of a set of parallel vertical rods having slots or notches evenly spaced along each of the rods. These notches are arranged in mutual alignment with the notches on the other rods to define a stack of shelves on which the wafers are robotically placed for processing. The spacing between the slots maintains the wafers at a sufficient distance from each other so that the wafers can be simultaneously exposed to a process gas.
FIG. 1 is a simplified diagrammatic representation of a conventional boat 10 according to the prior art. The boat 10 includes a plurality of parallel vertical support members, for example, rods or legs 12, typically three or four in number, that are equally spaced from a central vertical axis 13 and vertically extend between a base 14 and a top plate 16. Alternatively, the base 14 and the top plate 16 may be ring-shaped. A plurality of slots or notches 18 is provided in each of the rods 12, mutually aligned with notches of the other rods and facing inwardly toward the central axis 13. As shown in FIG. 1, a plurality of wafers 20, only one of which is shown for simplicity, is supported horizontally in the slots 18 of rods 12 with their centers aligned on the central axis 13. In some applications, each wafer is first placed in a removable ring-shaped susceptor, which is supported in the notches, particularly in applications in which wafer temperature uniformity is critical. However, such susceptors increase handling and decrease throughput, so that direct support of the wafers in notches on the support members is often preferable.
FIG. 1A is a cross-section through the boat 10 through one of the notches 18 in the legs 12, looking down from immediately above the wafer 20. It shows a wafer 20 of radius R, where R equals 100 millimeters (mm), 150 mm, or 225 mm, for example. The depths of the notches can be defined by a radius from the central axis 13 of the boat 10 to exceed the radius of the wafers 20 by about 5 mm or more, for example, providing a clearance C from the edge of the wafer 20 that is enough to insure that wafer 20 does not contact the legs 12 when being placed in or removed from the boat 10. This clearance C can be seen in the legs 12a and 12b of the three-legged boat 10.
Wafers 20, when in the boat 10, rest on the upwardly-facing bottom surfaces of the notches 18, thereby being supported at three areas around the perimeter of the wafer 20 (four areas for four-legged boats). Wafers 20 are inserted into and removed from the boat 10 from a front side of the boat 10, in the direction represented by the arrow 22, toward and away from the back leg 12a. Friction between the backsides of the wafers 20 and the upwardly-facing bottom surfaces of the notches 18 hold the wafers 20 in place during processing and while the boat 10 is being moved into or out of a reactor chamber.
Front legs 12b are positioned to the sides of the boat 10 to provide an opening at the front of the boat 10 that is larger than the diameter of the wafers 20, preferably allowing clearance C at both sides of the wafer 20. These front legs 12b support the wafers 20 forward of the center of gravity of the wafers 20. For example, the front legs 12b may be spaced at an angle A from a transverse centerline 24 of the wafers 20, through the center axis 13. This angle A may be, for example, 12.5 degrees, with the front legs 12b spaced an angle of 180-2A apart, or 155 degrees, for example. To provide the clearance C, the back wall of the slots 18b lie in a plane 26 that is spaced the distance C from the edge of wafers 20 at their transverse centerline 24. The slot 18a in the back leg 12a is parallel to a tangent to the edge of the wafer 20.
Wafers 20 are moved into and out of the boat 10 in groups of, for example, five wafers 20, as illustrated in FIG. 2. This loading and unloading of the boat 10 is carried out with a transfer robot 25 having a rotatable and extendable transfer arm 27 that is provided with a corresponding group of, for example five, end effectors 28, which move wafers 20 into and out of the boat 10 in the direction of the arrow 22. The transfer robot 25 typically transfers wafers 20 between the boat 10 and cassettes 30. To load and unload wafers 20 to and from all notches 18 of the boat 10, the transfer arm 27 of the transfer robot 25 may be vertically moveable, or the boat may be vertically moveable, or both, so that the end effectors 28 can align with any of the notches 18.
The above described boats 10 and transfer robots 25 are used for many processes. Depending on the process, the boats 10 can be coated with any of a number of substances. Carbon batch processes, for example, can be carried out on pluralities of wafers 20 held in a boat 10. In such carbon processes, carbon, often in the form of graphite, can coat the surfaces of the legs 12, including those of the notches 18. This carbon can act as a lubricant, reducing greatly the friction between the backsides of the wafers 20 and the upwardly-facing bottom surfaces of the notches 18. This lubrication allows wafers 20 to be moved transversely in the boats 10 at various stages of handling and processing. This movement, or wafer slippage, mis-aligns the wafers 20 such that, when picked up by a transfer arm 27, a wafer 20 can strike one of the legs 12, which can cause the wafer to break or will allow the wafer to slide out of the boat before transfer, which is highly undesirable and costly.
One solution to the problem of wafer slippage in a carbon process or other such process where such slippage tends to occur is to frequently clean the boat, even after each batch of wafers 20 is processed. This solution, while tolerable in a research or laboratory environment, impacts throughput in a commercial or production setting, which is excessively costly.
Accordingly, there is a need to solve the wafer slippage problem in batch processing boats in carbon processes and other processes where friction on wafer supporting surfaces declines during processing.